1. Field of the Invention
This invention relates generally to fiber optic receivers and transceivers and transponders, and in particular to an optical line-side with burst mode receiver and clock and data recovery and burst mode transmitter.
2. Description of the Related Art
Today's fiber optic based networks use transceivers or transponders at the interface between electronics and the optical signals that propagate on the optical fiber. A transceiver or transponder contains within it the basic elements of an optical transmitter, an optical receiver, and other electronics to perform clock and data recovery and physical layer protocol functions as well as functions to control these components. There are many applications for transceivers ranging from fiber to the home to data centers to long haul and high-performance communications. The performance of the transceiver or transponder as well as its cost is tied to the particular application. Today, transceivers are manufactured in a form factor called a pluggable like an XFP or SFP package, or as a board mountable component.
Typical transceivers are designed for use with protocols (e.g. SONET, SDH, FDDI, Gigabit Ethernet, Ethernet, Fiber Channel) in which the incoming data is well behaved, continuous and not bursty. In these transceivers, the receivers are designed to extract both the clock and data from an incoming optical signal that is continuous and typically cannot handle burst mode or unframed packet modes of transmission and reception. In conventional applications, the incoming data stream is continuous so it is acceptable for the clock and data recovery (CDR) module to take a long time to recover the clock. For example, the CDR design may be based on a phase-locked loop that takes some time to acquire phase lock. In these cases, if the CDR takes a long time to lock the clock, then it typically will also be a long time before the CDR loses the lock.
However, new burst mode applications may have packets that can consist of short packets, long packets or wide ranges of packet or burst lengths with unknown or non-determined inter packet or inter burst spacing. This new burst environment will require locking of the clock in a much shorter time, for example within approximately 32 bits or less. Typically, a CDR that is fast enough to lock the clock within 32 bits will also lose lock if the incoming signal is lost (or is no longer present) after 32 bits. For bursty, asynchronous traffic, this may happen quite often. It should be noted that the same hardware designed for burst mode operation can often also be used for framed, continuous or other types of data protocols and transport.
This mode of operation is not possible with current technology since conventional framed protocols trade off synchronous operation for clock lock time and clock hold time. As an example of a bursty-traffic environment, the communication system might use variable length, asynchronous packets with small overhead (e.g., short preambles) for clock and data recovery. The arrival time and optical power of the packets or bursts can be random and vary per packet or per burst. In conventional transceivers, stringent requirements are imposed onto the receiver electronics, and in particular on the CDR, when synchronous protocols such as SONET, SDH and Gigabit Ethernet are used. These protocols with synchronous clocked data typically require on the order of a couple of thousand transitions to lock, which at 10 Gbps in general means a couple of milliseconds.
Thus, there is a need for optical receivers that can more quickly lock onto a clock and hold that clock for a sufficient period of time, for example to accommodate bursty and/or asynchronous traffic.